Fix multiprocessing support for Windows binary

[allhailwesttexas]

The black binary generated for Windows builds would fail on reformatting multiple files due to a Windows-specific multiprocessing issue. Fix by calling freeze_support() as described in Python docs.

[allhailwesttexas]

I didn't create an issue for this, not sure if that's a required part of the workflow? If that's preferred, let me know and I'll do it and link this PR to it.

[zsol]

You probably want to add this in blackd.py as well

[coveralls]

Pull Request Test Coverage Report for Build 835

• 2 of 3 (66.67%) changed or added relevant lines in 2 files are covered.
• 140 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-4.5%) to 91.614%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
black.py	1	2	50.0%

Files with Coverage Reduction	New Missed Lines	%
tests/test_black.py	67	91.96%
blackd.py	73	8.75%

Totals
Change from base Build 824:	-4.5%
Covered Lines:	2775
Relevant Lines:	3029

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💛 - Coveralls

[allhailwesttexas]

@zsol Added now. At the moment, this binary is not built in CI, but I built it locally with C:\\Python36\\python.exe -m PyInstaller --clean -F --add-data blib2to3/;blib2to3 blackd.py and verified that it fails before the fix and works after.

[zsol]

Thanks!